[objc-arc] Fix assertion in EraseInstruction so that noop on null calls when passed null do not trigger the assert.

The specific case of interest is when objc_retainBlock is passed null.

llvm-svn: 185885

llvm/lib/Transforms/ObjCARC/ObjCARC.h

@@ -286,7 +286,9 @@ static inline void EraseInstruction(Instruction *CI) {
 
   if (!Unused) {
     // Replace the return value with the argument.
-    assert(IsForwarding(GetBasicInstructionClass(CI)) &&
+    assert((IsForwarding(GetBasicInstructionClass(CI)) ||
+            (IsNoopOnNull(GetBasicInstructionClass(CI)) &&
+             isa<ConstantPointerNull>(OldArg))) &&
            "Can't delete non-forwarding instruction with users!");
     CI->replaceAllUsesWith(OldArg);
   }